Low dropout regulators (LDOs) are used in many electronic devices today, especially portable electronic devices. For example, LDOs are often used in driving signals in touch sensing controllers of touch sensing systems of display panels. In such applications, the LDOs typically should work with large capacitive switching load (e.g., 100 pF) with series resistance (e.g., 1 k Ohm) at high frequency (e.g., 400 kHz). The capacitive load is usually replicated multiple times (e.g., 32) so that multiple rows of signals can be driven across a display panel. The RC-load is panel dependent and varies from one panel to another depending upon the touchscreen sensor technology used. Because the capacitive load varies widely and the load replicated multiple times depending upon sensing mode, a single closed loop LDO solution cannot be implemented due to stability issues.
A conventional solution is to provide a LDO with a single close loop having a source follower and an operational amplifier and a plurality of source follower replicas. FIG. 1 shows a conventional LDO usable in a touch sensing controller driver. The LDO 100 includes an operational amplifier 110, a source follower block 120, and a plurality of replica source follower blocks 130. To avoid obscuring the illustration, only one (1) replica source follower block 130 is shown. However, it should be appreciated that there are multiple identical replica source follower blocks 130 in the signal driver 100 (for example, 32 replica source follower blocks). The source follower block 120 includes a source follower device M1 and a bias device M2 connected to each other in series. Likewise, the replica source follower block 130 includes a source follower device M1r and a gain device M2r connected in series. The gates of both M2 and M2r are driven by a bias signal IBIAS.
As shown in FIG. 1, the gates of both M1 and M1r are coupled to an output of operational amplifier 110. A positive input of operational amplifier 110 receives a reference voltage VREF. The source of M1, which is coupled to the drain of M2, is feedback to a negative input of operational amplifier 110 to form a close loop. Thus, the voltage at the source of M1 is designated as VOUT_CL. In contrast, the source of M1r, which is connected to the drain of M2r, is not feedback to operational amplifier 110. The voltage at the source of M1r is designated as VOUT_OL. VOUT_OL is the output voltage of LDO 100. Thus, the driver using LDO 100 is referred to as “open loop” because the output load is not in the feedback loop formed by M1, M2, and operational amplifier 110.
One problem with the open loop driver is its size. Low output impedance is needed to charge and discharge the capacitive load in time (e.g., in less than 1.25 μsec). So, the source follower topology would result in very large devices (e.g., M1r in FIG. 1) since the output impedance is 1/gm. A large M1r can lead to significant power loss. To reduce the size of the driver, some conventional drivers have adopted a flip voltage follower (FVF) topology that allows the use of smaller devices.
FIG. 2 shows one example of a conventional low-side LDO with an FVF output stage usable in a touch sensing controller driver. A supply-side LDO with an FVF configuration could be constructed with a complementary design. LDO 200 includes an operational amplifier 210, a FVF 220, and a plurality of replica FVFs 230. To avoid obscuring the illustration, only one (1) replica FVF 230 is shown. However, it should be appreciated that there are multiple identical replica FVF 230 in the LDO 200 (for example, 32 replica FVFs). The FVF 220 includes a source follower device M1 and a gain device M2 connected in series. Likewise, the replica FVF 230 includes a source follower device M1r and a gain device M2r connected in series. The gates of both M1 and M1r are coupled to the output of operational amplifier 210. A bias device Mb is coupled between the drain of M1 and a power supply rail Vdd. Likewise, a bias device Mbr is coupled between the drain of M1r and Vdd.
The gate of M2 is coupled to the drain of M1. So M2 forms a tight local feedback loop in conjunction with M1 and Mb. Likewise, the gate of M2r is coupled to the drain of M1r. So M2r forms a tight local feedback loop in conjunction with M1r and Mbr. A positive input of operational amplifier 210 receives a reference voltage VREF. An output of operational amplifier 210 drives the gate of M1, while the source of M1 is feedback to a negative input of operational amplifier 210, thus forming a closed loop. In contrast, the source of M1r is not feedback to the operational amplifier 210. An output voltage is taken at the source of M1r. So the LDO 200 has an open loop configuration. Low output impedance is needed to charge and discharge the capacitive load in time (e.g., in less than 1.25 μsec). Using FVF topology as the second stage of LDO 200 results in an output impedance that is lowered by the loop gain. This will allow use of smaller devices, and hence smaller area.
However, one drawback of the FVF topology is the tight range of operation (typically 0.15-0.3 V) because Vgs−Vt (M2)>=Vdsat(M1)+Vdsat(M2) to ensure both M1 and M2 operate in saturation region. Achieving a large output range in LDO 200 would require setting M2 with a large Vgs−Vt, which requires using a very weak M2. This is counter-productive because M2 is providing all the load current in LDO 200. In sum, achieving large output range comes at a cost of degrading output impedance of LDO 200.
Therefore, there is a need in touch sensing controller design to provide a LDO with a wide tuning range without sacrificing the low output impedance.